Applications exist in which a system quantity to be monitored is characterized by relatively rapid changes that represent no useful information to the operator of the system. In those applications, the operator is only interested in long-term trends in the monitored quantity, and the rapid changes only serve to confuse the operator. An example of this is the brightness meter used in a recovery boiler. The brightness meter output is recorded on a strip-chart recorder, which may have a speed of around 1 inch per hour, and the trends to be detected develop over a range of, say, 1 to 2 hours. However, the output of the brightness meter, responding to variations in the shifting char bed, undergoes oscillations having periods of 10 to 20 minutes. These oscillations put confusing information onto the strip chart, and effective use of the strip-chart readout requires a great degree of interpretation.
In applications of this type and in others in which only long-term trends are of interest, it would be desirable to have a filter follow the sensor output so that only the useful information is displayed to the operator. However, filters for such applications would have long time constants, and realization of long-time-constant transfer functions is complicated by the fact that the energy-storage devices, principally capacitors, that conventional realizations call for are subject to levels of leakage that become significant when used in long-time-constant circuits.